Golf balls are generally classified into one-piece golf balls and multi-layered golf balls. The multi-layered golf balls include two-piece balls having a core coated with a resinous cover, three-piece balls having a core coated with a resinous cover through one or more intermediate layers, and thread-wound balls having a core wound with rubber thread and coated with a resinous cover. In the prior art, these one-piece golf balls and cores of multi-layered golf balls were prepared by molding a rubber composition comprising a rubber component (base rubber) such as polybutadiene, a co-crosslinking agent such as an unsaturated carboxylic acid metal salt, a filler such as zinc oxide, and a free radical releasing agent (co-crosslinking initiator) such as dicumyl peroxide while applying heat and pressure. These golf balls are known to show good flying performance as described in Japanese Patent Publication No. 19615/1980.
Although good flying performance is achieved with the use of the above-mentioned rubber composition, there is a room for further improvement with respect to the durability of one-piece golf balls and cores of multi-layered golf balls. Particularly in the case of one-piece golf balls having no cover thereon, the balls themselves have to be hard. Hardness can be increased by increasing the amount of an unsaturated carboxylic acid metal salt as the co-crosslinking agent in the rubber composition, but at the sacrifice of strength. Due to their insufficient impact resistance and durability, the balls are prone to cracking and crazing.